bug: constructor mutates it's argument (options)

[adrian-gierakowski]

for example here:

https://github.com/node-modules/agentkeepalive/blob/01e61e9e53526df2231152ef4287d38332f4db45/lib/agent.js#L32-L37

If the options argument is defined it should be cloned before having it's props mutated.

I'm happy to submit a PR, just wanted to check that the author agrees with the suggestion